In recent years, internal phone services in which mobile terminals, such as mobile phones, are used as internal company phones have been provided by telecommunications carriers. In general, in addition to a general mobile phone number, an internal phone number that is different from the general mobile phone number and that is used for an internal phone is assigned to such a mobile terminal that can use the internal phone service. By performing a call request operation using the internal phone number, a mobile terminal user can make a call using the internal phone without locating to a specific area in, for example, a company.
A communication system that implements such an internal phone service includes, for example, a mobile phone system that controls communication between mobile terminals and an internal phone system that controls communication between mobile terminals that are used as internal phones. In the mobile phone system, when an internal line call request including an internal phone number is received, the internal line call request is transmitted to an internal phone system. When the internal line call request is received from the mobile phone system, the internal phone system makes a call connection between the source mobile terminal that made the internal line call request and the called party's mobile terminal. By doing so, users of both mobile terminals can conduct an internal phone call.
Furthermore, if a call request is received with respect to a mobile terminal that is in a call, the mobile phone system provides a supplementary service, such as a voice mail service or a forwarding service that forwards an incoming call. Furthermore, if an internal line call request is received with respect to a mobile terminal that is in an internal phone call, the internal phone system provides a supplementary service, such as a voice mail service.    Patent Document 1: Japanese Laid-open Patent Publication No. 06-350522    Patent Document 2: Japanese Laid-open Patent Publication No. 2001-268227
However, with the technologies disclosed in the above Patent Documents described above, there is a problem in that, in some cases, the supplementary service may not be provided by the mobile phone system. For example, if the internal line call request with respect to the mobile terminal that is subjected to a call connection performed by the internal phone system, the supplementary service is not provided by the mobile phone system. Such a problem will be specifically described with reference to FIG. 14. FIG. 14 is a schematic diagram illustrating an example configuration of a conventional communication system.
As illustrated in FIG. 14, a conventional communication system 9 includes mobile management apparatuses 20 and 30 and an internal line managing apparatus 900. The mobile management apparatuses 20 and 30 control communication of a mobile phone 11 or a mobile phone 12. The mobile management apparatuses 20 and 30 are mobile phone systems each including a higher-level device, such as a base station or a base station control device. The internal line managing apparatus 900 is connected to the mobile management apparatuses 20 and 30 in a wired or wireless manner. The internal line managing apparatus 900 is an internal phone system that controls communication of the mobile phone 11 or the like when the mobile phone 11 is used as an internal phone.
With this configuration, in the example illustrated in FIG. 14, suppose that the mobile phone 11 and the mobile phone 12 are used as internal phones and are in a call connection (Step S91). In other words, suppose that the mobile phone 11 and the mobile phone 12 are subjected in a call connection performed by the internal line managing apparatus 900. In this state, as in the example illustrated in FIG. 14, a mobile phone 13 transmits, to the mobile management apparatus 30, an internal line call request for an internal phone call with the mobile phone 11 (Step S92). The mobile management apparatus 30 transmits the internal line call request received from the mobile phone 13 to the internal line managing apparatus 900 (Step S93).
Then, if the internal line call request is received from the mobile management apparatus 30, the internal line managing apparatus 900 determines whether the mobile phone 11, which is a called party, is in a call connection. In this case, because the mobile phone 11 and the mobile phone 12 are in a call connection, the internal line managing apparatus 900 provides the supplementary service. For example, the internal line managing apparatus 900 notifies, via the mobile management apparatus 30, the mobile phone 13 of a voice mail guidance (Steps S94 and S95).
In this way, if an internal line call request is transmitted to the mobile phone 11 that is subjected to a call connection performed by the internal line managing apparatus 900, the supplementary service is provided by the internal line managing apparatus 900. In other words, if a user of, for example, the mobile phone 11 receives an internal phone call from another mobile phone while the user is conducting an internal phone call, the user may not receive the supplementary service provided by the mobile management apparatus 20. There may be a case in which the supplementary service provided by the mobile management apparatus 20 or 30 is better than the supplementary service provided by the internal line managing apparatus 900. Accordingly, some users may want to receive supplementary service provided by the mobile management apparatus 20 or 30.